The present invention relates to the field of electronics, and, more particularly, to a radio frequency (RF) transceiver.
An RF transceiver, such as a cellular telephone, includes a transmitter and a receiver. Cellular telephones are typically full-duplex systems, that is, they can simultaneously transmit and receive voice and/or data information. The transmitter operates over a first assigned frequency band, and the receiver operates over a second assigned frequency band. The transmitter and receiver are typically connected to and share a common antenna through a duplexer.
RF signals received by the antenna are directed to the duplexer, which allows the desired signals to pass through to the receiver. These received signals are amplified by a low noise amplifier (LNA) before down conversion to an intermediate frequency (IF). Unfortunately, the duplexer may not provide sufficient attenuation to undesired signals. Consequently, some of the undesired signals are applied to the input of the LNA, even though at a reduced level.
One source of the undesired signals is from the transmitter colocated with the receiver in the housing of the cellular telephone. For example, a modulated signal from the transmitter may be coupled to the receiver through the duplexer. Another source of the undesired signals is from the transmitters of other cellular telephones or base stations operating nearby. An undesired signal generated by a transmitter external to the radio transceiver is typically referred to as a jammer signal.
Unfortunately, the undesired signals can interact with each other within the receiver through a process known as cross-modulation to produce a new interfering signal at the same frequency as the desired received signal. This problem becomes worse as the ratio of the undesired signals to the level of the desired signals increases.
Cross-modulation distortion can thus develop in the presence of two or more interfering signals. The two or more interfering signals are separated from the assigned input signal frequency and from each other such that the Nth order mixing of the interfering signals that occurs in a nonlinear device in the receiver produces a higher order intermodulation distortion product whose frequency is within the assigned input frequency band. The transfer functions of electronic devices commonly used in amplifying and mixing circuits within the receivers are seldom, if ever, perfectly linear. In other words, the non-ideal characteristics inherent in these devices leads to cross-modulation and intermodulation distortions.
A typical non-ideal linear element in the receiver is the LNA. Assuming the input stage of the LNA meets input linearity requirements, it is desirable that the output stage of the amplifier not distort the output signals due to voltage or current limitations of the amplifier. That is, if the output of the LNA includes desired and undesired signals, they must both be fully amplified. Consequently, this puts a higher demand on the LNA in terms of supply voltage and/or current headroom requirements.
The need to reduce cross-modulation distortion is further emphasized since cellular telephones are being designed to operate with ever lower supply voltages. For example, current cellular telephones are designed to operate with a power supply having a range of about 2.7 to 3.3 volts. This level was previously in the 5 volt range. Therefore, if the undesired out-of-band signals were reduced or eliminated, then the LNA could operate at lower supply voltages since it need only amplify the desired signals.
One well known form of non-linear distortion is a third order intermodulation distortion product which is directly related to cross-modulation. A 1 dB change in signal strength of the interfering signals results in a 3 dB change in signal strength of the undesired third order intermodulation distortion product. One approach to reduce the third order intermodulation distortion product is to increase the bias current of the LNA. However, portable cellular telephones obtain their power from portable power sources. These portable cellular telephones are thus designed to minimize power consumption to get maximum use in either a low current standby mode, i.e., when the cellular telephone is waiting for an incoming call, or a high current active use mode, i.e., when the cellular telephone is receiving speech or data. Increasing the current gain in the receiver to reduce intermodulation distortion is not desirable because the increased current drain reduces the amount of time that the cellular telephone can be used.
Another approach to reducing the third order intermodulation distortion products is to decrease the gain of the receiver front end. Radio designers typically refer to the stages in the receiver closest to the antenna as the front end and the stages furthest from the antenna as the receiver back end. Traditionally, the receiver front end gain is set sufficiently high to overcome the worst case receiver back end noise figure to achieve acceptable sensitivity. Typically, an LNA having a fixed gain is the first active stage in the receiver front end. The gain of the LNA is set high for a minimum receiver noise figure resulting in acceptable receiver sensitivity.
The penalty for high LNA gain, however, is linearity. As the LNA gain increases, the stages following the LNA, such as the downconverter, must be made more linear to maintain the same intermodulation performance. Unfortunately, higher linearity typically requires higher DC power dissipation which is undesirable for battery-operated cellular telephones. Conversley, if the LNA gain is lowered to improve intermodulation performance, the receiver sensitivity degrades.
A number of other techniques have been developed to attempt to reduce third order intermodulation distortion. For example, U.S. Pat. No. 5,758,271 to Rich et al. discloses a code division multiple access (CDMA) radio receiver wherein the gain of the radio receiver is adjusted responsive to the quality of the received signal to optimize the quality of the received signal. Since the adjusted gain also changes a received signal strength indication (RSSI) of the received signal, the RSSI of the received signal is estimated and compensated responsive to the gain of the radio receiver to produce a compensated RSSI of the received signal indicative of the RSSI of a desired RF signal.
U.S. Pat. No. 5,588,020 to Schilling discloses a spread spectrum CDMA communication system for communicating data between a plurality of personal communications network (PCN) users. A comb notch filter is connected to the output of a low noise amplifier via a downconverter. The comb notch filter notches the predetermined channels of the mobile cellular system for reducing the combined interfering power levels for mobile cellular users with a PCN base station.
U.S. Pat. No. 5,398,004 to Kobayashi discloses a wideband low noise amplifier having a first and a second feedback path. The bandwidth of the first feedback path is on the order of 2 GHz while the bandwidth of the second feedback path is on the order of 5 GHz. A resistor in the second feedback path is selected to optimize noise match, provide gain-bandwidth adjustment, and DC bias stabilization. The second feedback resistor also provides RF shunt feedback which can be adjusted to determine the gain-bandwidth response and input matching to the system performance. Kobayashi also discloses an alternative embodiment that includes the addition of an inductor connected in series with the second feedback resistor. The inductor is selected to provide an inductive reactance component that helps to further match the effective reactant""s component that is exhibited by a given system input.
U.S. Pat. No. 5,557,641 to Weinberg discloses a CDMA communication system having a charged coupled device (CCD) component performing a variety of functions, including filtering. The CCD provides signal processing to guard against adjacent channel interference. The CCD is a tapped analog delay line. The filter characteristics of this device are determined by the tap weights in the CCD.
U.S. Pat. No. 5,678,213 to Myer discloses a low noise amplifier in a CDMA cellular system having a plurality of bandpass filters. One of the filters is a bandpass filter with a passband of nominal width for receiving an input signal provided by the antenna. The other filters isolate the desired carrier signal from the spurious signals in the mixed down signal. The bandwidth of these filters depend on the operating system in which they are intended to operate.
In view of the foregoing background, it is therefore an object of the present invention to provide a radio transceiver, such as a cellular telephone, having enhanced cross-modulation performance.
It is another object of the invention to provide such a radio transceiver without placing a higher demand on the LNA in terms of supply voltage and/or current headroom requirements.
These and other objects, features and advantages in accordance with the present invention are provided by a radio transceiver comprising a receiver and a colocated transmitter. A duplexer may be connected to an output of the transmitter and to an input of the receiver and an antenna, which may couple undesired signals from the transmitter to the receiver. The receiver includes an LNA comprising a nonlinear portion capable of generating undesired cross-modulation signals based upon a portion of the transmit signal coupled thereto from the duplexer and a signal from another nearby transmitter. A bandpass filter may be connected to an output of the LNA, and at least one downconverter stage is connected to an output of the bandpass filter. A reactive termination circuit is preferably connected between the LNA and the bandpass filter for changing an impedance presented to the output of the LNA to thereby reduce undesired cross-modulation signals.
The reactive termination circuit preferably operates at the frequency of the receive signal coupled to the receiver via the duplexer. This signal is typically an amplitude modulated (AM) spread spectrum signal. According to one important feature of the invention, the reactive termination circuit terminates an output transresistance stage of the LNA into an AC short circuit at the frequency of the colocated transmitter signal, and terminates the in-band signal from an adjacent transmitter in a resistive termination. This significantly lowers the linearity requirements of the output transresistance stage of the LNA.
The reactive termination circuit is preferably a pi network. The pi network includes an inductor connected between a pair of capacitors. Cross-modulation performance of the LNA is improved for out-of-band signals without putting a higher demand on the LNA in terms of supply voltage and/or current headroom requirements.
The radio transceiver may be a full-duplex system, and is preferably a dual mode Code Division Multiple Access (CDMA)/Advanced Mobile Phone System (AMPS) cellular telephone. However, the reactive termination circuit is applicable to any radio transceiver subject to cross-modulation distortion. Since a cellular telephone typically operates from a portable power source, excess power consumption is to be avoided to prolong operation. The radio transceiver preferably operates from a battery having a range of about 2.7 to 3.3 volts.
Another aspect of the invention relates to a method for improving cross-modulation performance of the LNA to out-of-band signals. The LNA comprises a nonlinear portion capable of generating undesired cross-modulation signals based upon a portion of a transmit signal from the radio transceiver coupled thereto and a signal from another adjacent transmitter. The method preferably includes the step of connecting a reactive termination circuit to the LNA for changing an impedance presented to the output of the LNA to thereby reduce undesired cross-modulation signals therefrom.